A radiographic phosphor panel contains a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-tight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conventional screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible light from irradiated phosphor crystals.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
Degradation of final images due to panel discoloration has long been recognized for intensifying screens. There has not, however, been agreement as to the source of that discoloration. What has been noticed is that screens subject to prolonged exposure to photographic film have tended to become discolored. U.S. Pat. Nos. 4,374,905 and 4,360,571 state that the discoloration is due to "volatile organic constituents escaping from the associated photographic film" (U.S. Pat. No. 4,374,905, column 1, lines 40-59 and U.S. Pat. No. 4,360,571, column 1, lines 46-64). Great Britain Patent Application No. GB 2 017 140 A states:
"[I]t has been discovered that screens containing lanthanum-oxy-halide phosphors tend to discolor rapidly when in use and in particular when held in contact with an X-ray film, . . .
"Gadolinium-oxy-halides are similar . . .
"In spite of intensive research into this discolouration defect the cause of it is not yet clearly known but it appears to be a complex reaction caused, in part at least, by the hydroscopic nature of the lanthanum-oxy-halide phosphors or gadolinium-oxy-halide phosphors, the nature of the binder and the presence of the X-ray film held in contact with the screen for a period of time.
"Furthermore, under somewhat different conditions of use X-ray screens and in particular X-ray screens which contain lanthanum-oxyhalide or gadolinium-oxyhalide phosphors can lose speed due to a different defect which appears to involve only the phosphor. This is hydrolysis of the phosphor which is caused by water present in the phosphor layer due either to atmospheric moisture or aqueous cleaning fluid penetrating the protective layer of the screen. It is thought that quantities of halide or more surprisingly, the free halogen, released by hydrolysis may actually catalyse the discolouration of the binder or of compounds having migrated from the film." (page 1, lines 14-33)
U.S. Pat. No. 4,374,905, to Rabatin, teaches a solution to both discoloration by "volatile organic constituents" and attack by water. The phosphor for an intensifying screen was milled with anhydrous MgSO.sub.4 or ZnSO.sub.4 (0.5 to 4 weight percent) during preparation of the screen. It was proposed that the protective action was based upon the reaction: EQU MgSO.sub.4 +2 HOH.fwdarw.Mg (OH).sub.2 2 H.sup.+ +SO .sub.4.sup.2-.
U.S. Pat. No. 3,836,784, to Bates et al, teaches that small amounts of "stabilizers", such as sodium thiosulfate or potassium thiosulfate can be included in the fluorocarbon binder of an intensifying screen. Bates et al, which used an iodide containing phosphor, noted:
"[A]ctivated iodide phosphors are extremely hygroscopic. Absorption of small amounts of water rapidly reduces the conversion efficiency to a vanishingly small value. In order to employ activated iodide phosphors it is therefore necessary to provide the activated iodide phosphor in the screen in a form in which it remains stable for long periods of time.
"Various ways have been taught for using thallium activated potassium iodide and protecting the iodide from moisture." (Bates et al, column 1, lines 20-30)
U.S. Pat. No. 3,023,313 to De La Mater et al teaches the use of small amounts of sodium thiosulfate or potassium thiosulfate in the binder of an intensifying screen. Examples list 2 grams and 6 grams of sodium thiosulfate per 200 grams of potassium iodide phosphor.
In U.S. Pat. No. 4,360,571, to Rabatin, phosphors were treated with fatty acids or metal salts of fatty acids to prevent discoloration by "volatile organic constituents" and attack by water. In GB 2 017 140 A, intensifying screens were stabilized against discolouration and hydrolysis by incorporation of a compound containing a free epoxy group and, optionally, a dialkyl tin compound such as dibutyl tin dioctyl as an additional stabilizer.
Radiation image storage panels, unlike intensifying screens, are subject to degradative losses of both emitted light and stimulating radiation. Since these effects are cumulative, discoloration can be an even more serious issue in storage panels than in intensifying screens.
Yellowing of a phosphor layer of a radiation image storage phosphor panel, in which the phosphor contains iodine, is described in European Patent Specification No. EP 0 234 385 B1. The yellowing is ascribed to liberation of free iodine. The solution taught for the yellowing problem, is incorporation in the phosphor layer of a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and metal salts of organic acids, specifically: octylic acid, lauric acid, stearic acid, oleic acid, ricinoleic acid naphthenic acid, 2-ethylhexanoic acid, resin acid, synthetic carboxylic acid, benzoic acid, salicylic acid, organic phosphinous acid, phenol, and alkylphenol.
It would be desirable to provide improved prompt emission and radiation image storage panels with stability against yellowing and improved methods for preparing radiographic phosphor panels.